Charge coupled devices (CCDs) are light sensitive elements that are formed on a semiconductor wafer. CCDs contain a plurality of photodetecting picture elements (pixels). The pixels can detect light and output an electrical signal in response to the light. The magnitude of the output electrical signal is indicative of the intensity of the light that reaches the pixel.
CCDs can sense light from a light source. The pixels in the CCD sense light and output signals indicative of the intensity of the impinging light rays. The output signals are stored in vertical shift registers located on the semiconductor wafer. The output signals are then read out of the CCD and used to produce an image of the object. CCDs are very sensitive to light. Therefore, the image produced can be a very accurate reproduction of the object. CCDs can be used to build an imaging device or a camera.
Previously known charge coupled devices have typically been too small to capture light from a large area. It would therefore be desirable to provide a large area charge coupled device that can provide image data at a frame rate fast enough for video images.
The present invention provides large area CCDs that provide image data at a frame rate fast enough to produce video images. Charge coupled devices of the present invention can have a large photo sensing area. The charge coupled devices have rows and columns of pixels (i.e., photosites).
Each photosite includes a pinned photodiode. The pinned photodiodes have a relatively high quantum efficiency. Also, the charge carriers in the pinned photodiodes can be depleted easily at normal operating voltages. This means that charge can be transferred out of the photodiodes at a fast frame rate with minimal residual charge left behind.
Each column of photosites in a charge coupled device (CCD) of the present invention has a corresponding vertical shift register. The vertical shift registers are located in between the photosites. A charge coupled device with alternating photosites and vertical shift registers is called an interline transfer CCD. Because the vertical shift registers are close to each photodiode, the charge only needs to be transferred a short distance from the photodiodes to the vertical shift registers. This feature also increases the frame rate.
The vertical shift registers store charge signals generated at the photosites. The charge signals are then transferred along the vertical shift registers and stored in horizontal shift registers. The vertical shift registers have doping gradients that facilitate the transfer of charge along the vertical shift registers. The doping gradients allow charge to be transferred along the vertical shift registers at a fast frame rate.